1. Field of the Invention
This invention concerns turret mooring systems for loading and offloading liquid petroleum product oil tankers, floating storage (FSO) vessels, floating production storage and offloading (FPSO) systems, floating vessels for natural gas offloading (for example, cryogenic liquefied natural gas (LNG) regas import terminals), and LNG transport vessels. Specifically, the invention is employed in a vessel with a mooring system having a tube or closed turret that is mounted inside the vessel hull by a bearing structure that can withstand axial and radial deformation forces from wind, waves and currents.
2. Description of the Prior Art
In the offshore hydrocarbon production industry, cost considerations have led to the use of floating, production, storage and offloading and similar vessels coupled to subsea wells via one or more risers. Turret-moored vessels are free to weathervane, and they use one or more production swivels to couple shipboard conduits to the fixed risers that run between the swivels and the sea floor. Numerous configurations of vessels moored by internal turrets or tubes are known in the art.
The dynamic weather, sea states, tides, and currents cause the vessel to pitch, roll, heave, fall and rotate, and the resultant forces are transferred to the mooring system. The environmental forces acting on the vessel cause the vessel to weathervane because the turret is axially and radially supported from the vessel by one or more bearing assemblies. During pitching, yawing and rolling of the vessel under the influence of the wind, waves, and currents, the hull of the vessel bends and twists, possibly deforming the bearing assemblies and thus hampering free rotation of the vessel.
In addition to force considerations, the size and structure of turret bearings are affected by the number and size of the risers or conduits which must pass through the turret. Although FPSOs were originally conceived to exploit smaller oil fields and to convert an existing small or medium size oil tanker to a FPSO, there is a trend to connect as many subsea wellheads as possible to FPSOs. As a result, very large tankers are being converted to turret-moored FPSOs. A floating vessel is not a static structure but rather the structure is subjected substantial bending and twisting. Therefore, a bearing assembly rigidly connected to a large vessel may be subjected to great oval shaped deformations.
In response, turrets employing non-precision bearings designed with adequate “play” to accommodate such deformations have been used. For example, U.S. Pat. No. 3,440,671 issued to Smulders provides within the hull of a vessel a cylindrical hollow chamber, in which is disposed a moored, buoyant tubular element with sets of wheels fitted at intervals along its periphery and arranged to roll within roller tracks within the chamber, thus permitting turning of the vessel with respect to the tubular element. The cylindrical chamber may become deformed to an oval shape under certain load conditions. However, because the self-adjusting sets of wheels provide sufficient play to tolerate such deformation, the bearing arrangement permits rotation even when deformation of the cylindrical tube walls occurs.
Although non-precision bearings accommodate deflection, they typically have more friction than corresponding precision ball or roller bearings (when not deformed by hull loading). If precision ball and roller bearings are used in a mooring configuration such that they are not subject to deformation, it is possible for the vessel to turn with minimum resistance about the geostationary turret tube. Due to deformation, however, larger, more robust precision bearings are typically required, with resulting higher cost.
U.S. Pat. No. 5,266,061 issued to Poldervaart et al. discloses an arrangement designed to reduce the deformation forces applied to the turret bearing, and thus to allow smaller less expensive precision bearings to be used. The Poldervaart et al. arrangement is shown in FIG. 1 of the drawings appended hereto. The vessel (1) is equipped with a vertical cylindrical tube (3) forming a hollow chamber extending substantially the height of the vessel from the keel (7) to the main deck (5). The upper portion of the tube (3) is fixed to the hull (2). The hull (2) includes a moon pool (9) into which a downwardly projecting lower end portion (11) of tube (3) extends a substantial distance. The hull (2) joins the tube (3) above the lower end (11) of the tube (3) and closes the top of the moon pool (9) as shown in FIG. 1. The lower end (11) is spaced away from the side walls (8) of moon pool (9) and from the hull (2) of the vessel (1).
A rigid ring (13) is secured to the inside of the lower end of lower end (11) of tube (3). A vertical rotary tube or turret (15, 16) having a lower portion (15) and an upper portion (16) is supported on rigid ring (13) by an axial/radial bearing (17). The bearing (17) has an outer race secured to ring (13) and an inner race secured to lower portion (15) of the turret (15, 16). At its lower end, lower portion (15) of turret (15, 16) carries a chain table (19) to which are secured anchor chains (21), by which the vessel is moored to the sea floor. Turret (15, 16) is hollow from top to bottom. Conduits (23, 25) extend vertically therethrough to serve as gas and hydrocarbon product risers and the like.
In operation, the vessel (1) is moored to a relatively fixed position by anchor chains (21) so that gaseous and/or liquid products can rise through conduits (23, 25) for storage onboard the vessel (1) or transportation to shore. Despite the location of the mooring system generally amidships, which subjects it to minimum influence from pitching and yawing of the vessel, the fixed tube (3) may deformed where it is connected to the hull. An undeflected fixed tube (3) is shown by the horizontal cross section of tube (3) in FIG. 2, with the dashed lines (3′) representing ovaling deformation of tube (3). The deformation forces are not transmitted to rigid ring (13), because the downwardly projecting portion (11) of fixed tube (3) between the hull and ring (13) is not secured to the hull and is therefore free to deform at its upper end while not significantly affecting its lower end. The rigid ring (13) holds the lower end of projection (11) against deformation and, more importantly, prevents deformation of the outer bearing race of bearing (17). The inner race of bearing (17) is not subjected to deformation by hull stress because it is not connected to the hull other than by the outer ring of bearing (17).
While the Poldervaart et al. arrangement serves to isolate the turret bearing from hull deformation, the hull of an ordinary ship requires extensive vessel structural modifications to construct a moon pool (9) in order to install the mooring system, thus adding cost.
3. Identification of Features Provided by Some Embodiments of the Invention
A primary object of the invention is to provide a vessel with a mooring arrangement having a precision bearing arrangement that is isolated from stresses of hull deformation.